Ulnar collateral ligament (UCL) injuries of the thumb sometimes occur with avulsion fracture. Avulsion fractures generally require open reduction and internal fixation to restore stability of the metacarpophalangeal (MP) joint of the thumb. Although several surgical techniques have been reported, UCL avulsion fractures are still challenging hand injuries to treat and to obtain stable fixation that allows early finger motion because of small size of fragment. This report describes two cases of thumb UCL avulsion fractures that were successfully treated with two-hole mini hook plates by adapting 1.5-mm variable angle locking hand plates. This method allowed for early postoperative rehabilitation and provided stability of the thumb MP joint and satisfactory clinical outcomes. Altogether, hook plate fixation could be an alternative technique for thumb UCL avulsion fracture.